I purchased 2020 AT4 6.2L 14 days ago - I drove it for 6 days before going back into the shop. I didn't think anything about it, but after reading your post - I realized I had the same issue. It would go into Reverse/Drive, but showed the P Brake was on, even though I hadn't put it on. It's only happened once.
My issue is - after the truck sat for a full day with the keyfob in the truck, I went to go start it and it wouldn't start. The dash came on and said "Keyfob left in vehicle" and when I went to start....NOTHING. No power, wouldn't even turn over. I stepped outside with the FOB thinking I could remote start the truck and, nothing. Had to connect jumper cables, to get it to start. After it started, check engine light came on and it reset everything in the truck, put it into "DEMO MODE". I've since dropped it off at the dealer, and it's been there a week......they said the code was a transmission code. Super frustrated.....